Apparatus for the production of thin walled plastic articles by deep drawing thermoplastic or light material in foil or strip form are known in the art. See, for example, U.S. Pat. No. 3,161,915 patented Dec. 22, 1964. In a thermoforming apparatus the article, once formed, is extracted from the dies of the thermoforming apparatus either using mechanical means or by applying a negative pressure to engage the articles. The mechanical means are not completely satisfactory in that the mechanism is normally quite complex and slower in operation than desired in a commercial machine. A transfer mechanism for applying a negative pressure to formed articles is disclosed in the aforesaid U.S. Pat. No. 3,161,915. The transfer mechanism comprises a carriage bearing on its underside a porous plate with a vacuum chamber above the porous plate. In operation, vacuum is maintained on the plate by means of a control valve to engage formed articles as best shown in FIG. 19 of the '915 patent. As shown, a vacuum is applied to the base or bottom of the article being formed. The dies of the thermoforming apparatus are then fully separated and the extractor plate carries the formed articles to a discharge conveyor or the like. This device is not completely acceptable in that the formed articles are disengaged from the extractor plate by releasing vacuum in the suction chamber. This takes time and, accordingly, is a limiting factor in the speed of operation of the machine.
U.S. Pat. No. 4,560,339 issued Dec. 24, 1985 describes a thermoforming apparatus in which articles are formed and cut in one unit and are extracted from the unit utilizing an extractor comprising a movable plate having a vacuum chamber similar to that shown in U.S. Pat. No. 3,161,915. However, in the '339 patent the lower face of the vacuum chamber, i.e., the face contacting articles to be removed, utilizes a plurality of inter-related channels and holes for applying adjustable suction and compression by connection to a vacuum and compressed air unit in order to extract the articles being formed from the recesses of the dies and to transfer then to an article stacker. The unit disclosed in the '339 patent again has disadvantages. Thus, U.S. Pat. No. 4,872,826, assigned to the assignee of the '339 patent and having the same inventor, in reference to the British equivalent of United States patent, states that the negative pressure used to extract the articles is brought about in accordance with the known art by adjusting the suction and shutting it off upstream of the suction holes provided on the extractor. It is stated that it is essential to have an optimum seal between the article formed and the plate. In addition, it is further stated that it has proved particularly difficult to obtain low negative pressures using the known art.
Adjustment and shutting off of the suction upstream of the suction holes in the extractor plate, again as stated in the '826 patent, does not cause formed articles to adhere to the extractor plate by means of air suction. Thus, the suction must be increased in order to draw the formed articles towards the extractor plate, with the result being that when the formed articles impact upon the extractor plate a considerable negative pressure is created within the articles, which is prejudicial to the integrity of the thin walls of the article.
The invention of the '826 patent, as stated therein, provides an apparatus for obtaining a negative pressure in formed articles and a flat extractor plate supporting the formed articles. The apparatus comprises a flat extractor plate for engaging formed articles which when resting their rims on the flat surface of the extractor plate form chambers, wherein a suction unit is connected with the formed chamber, by means of a pipe and a connecting cavity and wherein a vacuum shut-off valve is provided and an air intake adjustment valve is inserted between the suction source and the shut-off valve, and wherein each chamber is provided with at least one intake for atmospheric air into the chambers.
Although U.S. Pat. No. 4,872,826 describes a device which proportedly solves the problem associated with the device of U.S. Pat. No. 4,560,339 the device is complex in the requirement for a plurality of inter-related channels and holes. As is apparent from the '826 patent, particularly FIGS. 1-3, a hole must be positioned substantially in the center of a channel with that hole being substantially centered in the article formed by the thermoforming apparatus to form a chamber. Moreover. the use of a separate air intake adjustment valve in conjunction with the suction unit adds complexity and cost to the overall device.
Accordingly, there is a need for an improved extractor plate for removing thin walled articles from the dies of a thermoforming apparatus which permits control over the amount of suction so as to not collapse the articles, and which permits rapid release of the formed articles once the extractor plate is out of engagement with the dies of the thermoforming machine.